A microgrid is a localized grouping of electricity generation, energy storage, and loads that normally operates connected to a traditional centralized grid (macrogrid) via a point of common coupling (PCC). This single point of common coupling with the macrogrid can be disconnected, islanding the microgrid. Microgrids are part of a structure aiming at producing electrical power locally from many small energy sources, distributed generators (DGs). In a microgrid, a DG is connected via a converter which controls the output of the DG, i.e. the current injected into the microgrid.
A microgrid (in grid connected mode, i.e. connected to the macrogrid) supplies the optimized or maximum power outputs from the connected DG sites and the rest of the power is supplied by the macrogrid. The microgrid is connected to the macrogrid at a PCC through a controllable switch. This grid connection is lost during grid fault and the microgrid is islanded.
During islanding, there is a risk of imbalance in the microgrid due to the loss of power import from grid as well as loss of voltage control by the grid. For voltage control it is required to change control mode of the DGs. The power balancing is solved by fast storage action and immediate load shedding schemes.
In an AC microgrid, the frequency is the same everywhere in steady state while voltage may differ depending on the power flow. However, in a microgrid with a continuous change in DG output, load switching and low inertia, there is continuous frequency and voltage fluctuation to a small scale. And the deviations are larger during large transients (like DG fault etc.).
Frequency and voltage stability relates to minimum oscillations and overshoot with ability to come back to initial value (or any other steady state value within acceptable deviation) after a disturbance.
A microgrid with multiple DGs and loads requires several switches at different level to connect and disconnect different network parts within the microgrid as well as to the main power grid. While the automatic disconnections by these switches are aimed for system protection, planned connection and disconnections are required for optimized operation ensuring power balance and resynchronization maintaining acceptable system dynamics in voltage, frequency and power oscillations. Before reconnection of a network to the microgrid resynchronization with voltage magnitude, phase angle and frequency matching is performed to ensure stability of the microgrid at reconnection.
US 2007/129110 discloses the use of an interface switch to re-join an islanded part of an electrical power network with the rest of the network. An interface switch is closed when the voltage difference between E and V and the relative phase angle deltaEV between E (external voltage) and V (internal voltage) are both small, and when the higher frequency voltage (as between E and V) leads the lower frequency voltage. Thus, the document teaches to observe the voltages and phase angles to choose a good time for reconnecting the islanded network.